Dynamics of the airflow above breaking and non-breaking waves

The dynamics of the coupled ocean-atmosphere boundary layers are strongly influenced by the momentum fluxes at the air-sea interface. Wind waves simultaneously influence and are generated by the atmospheric forcing. In an attempt to better understand and quantify this complex feedback, we present laboratory measurements of the velocity in the airflow above wind-generated and mechanically generated waves. Experiments were performed in the large wind-wave flume at the University of Delaware's Air-Sea Interaction Laboratory. The tank is 42 m long, 1 m wide, and 1.25 m high, with a mean water level of 0.71 m. A wide range of wind speeds (U₁₀ from 0 to 13 m.s^-1) and wave frequencies were independently used, thus creating alternatively non-breaking, micro-breaking and breaking waves. Airflow velocities were obtained using PIV optical techniques, yielding direct measurements of the velocity in the airflow and within the viscous sublayer above the wavy surface. We intermittently observe a separation of the viscous sublayer past the crest of the wind-waves for wind speeds as low as 5 m.s^-1. Vorticity is transported away from the surface, generating significant mixing. Recurring separation leeward of the crests leads to substantial along-wave variabilities of surface viscous tangential stress and velocity. A wave-phase sensitive processing of the data showed a statistical correlation of these variabilities with surface wave slope parameters. These results, when coupled with proper surface wave spectra data, could lead to a parameterization of the effects of airflow separation on the total air-sea stress.